Conventionally, a fixing device, which is one of representative heating devices for use in such electrophotographic apparatus as a copying machine and a printer, generally and often has a configuration wherein heating means comprising a halogen heater or the like is disposed inside a fixing roller comprising a hollow core of aluminum or the like and the fixing device is heated to a predetermined temperature (fixing temperature) by causing the halogen heater to generate heat.
This configuration, however, involves a problem that a length of time from the start of heating until the temperature of the fixing roller reaches the predetermined fixing temperature, which is so-called warm-up time, is long and, hence, the fixing roller need be preheated even during standby for the purpose of improving ease of use, thus resulting in increased power consumption during standby.
To overcome this problem, attempts have recently been made to thin the wall of the fixing roller by using an iron material having a superior strength to aluminum for the fixing roller in order to lower the heat capacity of the fixing roller, thereby to shorten the warm-up time. In this case, however, the fixing roller has a lowered heat flow along the axis thereof, thus raising a problem that what is called “abnormal temperature rise at a sheet non-passage portion”, which is a phenomenon that if recording sheets of a size smaller than the fixing roller length pass through the fixing roller successively then the surface temperature of the fixing roller in a portion over which the recording sheets do not pass (sheet non-passage portion) rises abnormally, becomes easy to occur.
To solve such a problem, such a fixing device has been proposed that plural (mostly two) heaters for different heating regions are used to heat the different heating regions of the fixing roller selectively in accordance with the size of a recording sheet used (see patent document 1 for example).
Such a heating system employing plural heaters is basically classified into the following two types. The first type comprises a combination of a heater 234a for heating a central region and a heater 235c for heating the entire width region, as shown in FIG. 24(a). When a large-sized recording sheet is to pass through the fixing roller, only the entire width region heater 234c is actuated for heating. On the other hand, when a small-sized recording sheet is to pass through the fixing roller, only the central region heater 234a is actuated for heating.
In the first type system, however, end portions of the fixing roller 231 are not supplied with heat during successive passage of small-sized recording sheets and, hence, the temperature thereof is lower than that of the central portion. For this reason, if a large-sized recording sheet is passed immediately after passage of small-sized recording sheets, a problem of unsatisfactory fixing performance arises due to a fixing failure, wrinkling, curling or the like, which occurs in edge portions of the recording sheet.
The second type system performs heating by means of a heater (main heater) 234a for heating a central portion and a heater (sub-heater) 235a for heading end portions, as shown in FIG. 24(b). In this case, one temperature sensor 237 and one temperature sensor 238 are provided at the central portion and one end portion, respectively. The main heater 234a and the sub-heater 235a are each controlled based on a temperature detected by a respective one of the sensor 237 at the central portion and the sensor 238 at the end portion.
The second type system is capable of exhibiting satisfactory fixing performance even upon passage of a large-sized recording sheet immediately after passage of small-sized recording sheets without experiencing the aforementioned temperature drop at the end portions by controlling the temperature of the end portions of the fixing roller 231 to an appropriate temperature by means of the sub-heater 235a even during the passage of small-sized recording sheets.
Further, a method in relation to the second type system has been proposed such that a shortcircuiting stem is inserted into a filament coil in a no-heat generating section of each heater to prevent the no-heat generating section from generating heat, thereby further suppressing the abnormal temperature rise at the sheet non-passage portion during successive passage of small-sized recording sheets (see patent document 2 for example). Hereinafter, a heater lamp used in this type will be referred to as a partial lamp while a heater lamp used in the conventional type referred to as a normal lamp.
Table 1 and FIGS. 12 and 13 show the results of comparison between the case where normal lamps were used for both of the main heater and the sub-heater in a high-speed multifunctional machine having a printing speed of 70 cpm (pattern 1) and the case where partial lamps were used for both of the main heater and the sub-heater (pattern 8) as to temperature distribution along the axis of the fixing roller immediately after successive passage of 100 A4- or B5R-size recording sheets. In Table 1, MRnh represents a mean value of heat distribution in the no-heat generating section of the main heater and SRnh represents a mean value of heat distribution in the no-heat generating section of the sub-heater.
TABLE 1Pattern 1Pattern 2Pattern 3Pattern 4Pattern 5Pattern 6Pattern 7Pattern 8Main HeaterTypeNormal ANormal BNormal CNormal BPartial ANormal CPartial BPartial AMRnh (%)48.035.930.535.913.130.526.313.1Sub HeaterTypeNormalNormalNormalPartialNormalPartialPartialPartialSRnh (%)36.536.536.514.236.514.214.214.2Σ Rnh (%)84.572.367.050.149.544.740.527.2
Heat distributions of respective heater lamps are shown in FIGS. 9 to 11 and Table 1 in which “normal A” of FIG. 9 and “normal” of FIG. 11 correspond to the main heater and the sub-heater, respectively, of pattern 1 while “partial A” of FIG. 10 and “partial” of FIG. 11 correspond to the main heater and the sub-heater, respectively, of pattern 8.
As can be seen from FIGS. 12 and 13, the temperature uniformity obtained with respect to moderate A4-size sheets in the case where partial lamps were used for heater lamps (pattern 8) was comparable to that obtained in pattern 1 employing conventional normal type lamps, and pattern 8 substantially reduced the temperature rise at the sheet non-passage portion with respect to small B5R-size recording sheets as compared with pattern 1.
Patent Document 1: JP H8-220930A (paragraphs [0017] and [0018], FIGS. 1 and 2)
Patent Document 2: JP 2002-258646A (paragraphs [0015] to [0021], FIGS. 1 and 2)